I used Campbell Cohen, and they are very thorough when it comes to checking that my documents are correct. Any data that was in error or wasn't completely clear they would question and make me clarify. At times it was annoying because it delayed my sending of the application by a few days or so. However, I'm glad they did because I have never had any problems with my documents. I had 53 points going into my interview (only needed 2 points), so I'm not sure how helpful my interview experience is for most. Most of my interview was in English. My advice would be to convince the interviewer that you are putting some effort into learning French. Furthermore, it would be helpful to do some research about Quebec in general, especially relating to your area of training (job market, certifications/licensure, etc.).
Here's my interview experience:
http://www.canadavisa.com/canada-immigration-discussion-board/quebec-applicants-lets-share-experience-t37196.0.html;msg2942990#msg2942990